1. Field of the Invention
The present invention relates to an apparatus and a method for controlling a reverse rate in a mobile communication system, and more particularly to an apparatus and a method for limiting a rate of a mobile station to which an autonomous rate control is granted.
2. Description of the Related Art
In a conventional mobile communication system, a reverse data transmission from a mobile station (MS) to a base transceiver station (BTS) can be achieved through packet data channels in a physical layer packet unit. A data rate of each physical layer packet may vary depending on packets and the BTS controls the data rate of each physical layer packet. That is, the BTS controls data rates of various MSs. Such a procedure of the BTS for determining and controlling data rates of the MSs is called a “scheduling”. The BTS conducts the scheduling based on feedback information transmitted thereto from the MSs on the basis of information related to power of the MSs and quantity of data to be transmitted from the MSs. That is, a scheduler of the BTS conducts the scheduling by taking a load obtained from “rise of thermal (RoT)” or “signal to noise ratio (SNR)” of MSs located in a service area of the BTS into consideration.
The control scheme of the BTS for the reverse data rates of the MSs is mainly classified into a fast scheduling scheme and a rate control (RC) scheme.
An MS operating with the fast scheduling scheme transmits a rate request message including a present buffer state and information about usable power of the MS to the BTS. Upon receiving the rate request message from the MS, the BTS transmits rate grant information to the MS by taking thermal noise, QoS of the MS, and other relevant information into consideration so as to allow the MS to transmit data with a maximum rate.
In the fast scheduling mode, the MS can transmit the rate request message including a present buffer state and information about usable power of the MS to the BTS, and the BTS can allocate a specific rate to the MS through grant message information by taking a request of the MS and a load state of a cell into consideration.
The RC scheme of the BTS is classified into a DRC (dedicated rate control) scheme and a CRC (common rate control) scheme depending on transmission schemes of control information. The BTS controlling the MS using the DRC scheme may transmit dedicated rate control information to each MS located in the service area of the BTS, and the BTS controlling the MS using the CRC scheme may transmit common rate control information to each MS located in the service area of the BTS.
That is, the BTS capable of controlling the MS using the DRC scheme transmits dedicated control information to each MS located in a cell, so the BTS can finely control the rate MS as compared with the BTS using the CRC scheme wherein all MSs treated in the service area of the BTS are controlled in common. However, in the DRC scheme requires, a great amount of control information to be transmitted than using the CRC scheme.
A BTS capable of controlling an MS with the CRC scheme transmits control information notifying MSs located in a cell of a “Busy” state of the reverse transmission if the RoT measured by the BTS exceeds a predetermined limitation value, or transmits control information notifying the MSs of a “Not Busy” state of the reverse transmission if the RoT is lower than the predetermined limitation value. When the control information representing the “Busy” state is received in the MS from the BTS, the MS can reduce the RoT of the cell by lowering the data rate of the MS or a traffic-to-pilot ratio (TPR) which will be described later. In addition, when the control information representing the “Not Busy” state is received in the MS from the BTS, the MS can increase the data rate of the MS or the TPR. Such an information related to the “Busy” and “Not Busy” states can be transmitted to the MSs through a rate control bit (RCB) being one bit in size.
The DRC scheme of the BTS is classified into a full rate transition scheme and a limited rate transition scheme according to a transition degree of the reverse data rate of the MS.
According to the full rate transition scheme, the BTS controls the reverse data rate of the MS without limiting a transition range of the data rate. In contrast, according to the limited rate transition scheme, the BTS controls the reverse data rate of the MS while limiting the transition range of the data rate within one step.
For example, if a set of the data rates includes 9.6 kbps, 19.2 kbps, 38.4 kbps, 76.8 kbps, 153.6 kbps, and 307.2 kbps, the number and specific value of data rates included in the set of the data rates may vary depending on systems. According to the full rate transition scheme, one of the data rates included in the data rate set can be determined as the data rate of the next packet of the MS by means of the BTS. That is, according to the full rate transition scheme, the MS transmitting data with a data rate of 9.6 kbps can transmit the next packet with a data rate of 307.2 kbps at a time because the BTS allows the MS to transmit the data with a predetermined reverse rate regardless of a previous data rate of the MS.
In contrast, according to the limited rate transition scheme, the BTS may determine the data rate of the next packet of the MS while up-converting or down-converting the data rate from the previous data rate of the MS by one step. For instance, a MS transmitting data with the data rate of 76.8 kbps can transmit the next packet with the data rate of only 38.4 kbps, 76.8 kbps or 153.6 kbps. In other words, since the data rate of the MS is up-converted or down-converted from the data rate of 76.8 kbps by one step, the transition range for the data rate of the MS is limited. Commands used for up-converting, down-converting and holding the data rate of the MS can be represented as “UP”, “DOWN” and “HOLD”. In addition, a signal mapping into “+1”, “−1” and “0” can be performed.
The full rate transition scheme and the limited rate transition scheme each have advantages and disadvantages.
The full rate transition scheme has an advantage in that the BTS can determine the data rate of the MS without limitations. However, the full rate transition scheme has a disadvantage in that it requires a great amount of bits in order to transmit the scheduling result to the MS. For instance, if six data rates exist as described above, 3 bits are necessary for representing all data rates. In addition, since it is necessary to transmit information about identifiers of the MSs, a great amount of information must be transmitted. In contrast, the full rate transition scheme has a disadvantage in that an amount of interference exerting an influence upon other cells may significantly vary depending on the data rate of the MS, so that serious channel variation of the MSs located in other cells may occur, deteriorating the system. In addition, the limited rate transition scheme has a disadvantage in that the BTS must determine the data rate of the MS within a limited range. In contrast, the limited rate transition scheme may allow the BTS to transmit the scheduling result to the MS by using one bit, so an overhead thereof will be reduced. In addition, the limited rate transition scheme limits the transition range of the data rate of the MS within one step, so variation of the interference exerting an influence upon other cells is relatively reduced.
The BTS may set a maximum autonomous rate and a possibility of the maximum autonomous rate with respect to a specific service for the MS in order to reduce delay generated during the data rate control procedure. That is, if service data which can be autonomously transmitted are generated, the MS can transmit the data by selecting a predetermined data rate within a range of the maximum autonomous rate allocated thereto, so it is possible to minimize the delay. However, it is difficult to predict a point of time for the service data which can be autonomously transmitted. For this reason, the BTS must reserve resources corresponding to a sum of the autonomous data rates allocated to the MSs.
In the meantime, besides the above system in which the BTS controls the data rate of the MS, a system including a BTS capable of controlling a TPR” of an MS can be provided.
In the conventional mobile communication system, a reverse data transmission of the MS is power-controlled by means of the BTS. According to the power-control procedure for the MS, the MS receives a power control command from the BTS so as to directly control power of a pilot channel thereof, while controlling channels other than the pilot channel with a fixing value of the TPR. For example, if the TPR is 3 dB, a ratio of power of the traffic channel transmitted from the MS to power of the pilot channel is 2:1. Accordingly, when the MS determines a power gain of the traffic channel, the MS sets power of the traffic channel as a double of power of the pilot channel. Such a procedure is also available for other channels. That is, a gain of a corresponding channel is set with a fixed value in relation to a gain of the pilot channel. In a system in which the BTS controls the TPR instead of controlling the data rate of the MS, the BTS schedules the reverse transmission of various MSs while directly notifying the MSs of the scheduling result through the data rate so as to control the MSs. That is, the BTS notifies each MS of the TPR allocated thereto. The TPR may increase as the data rate increases. For instance, since an increase of the data rate by two causes the power allocated by an MS to the traffic channel to double, this increase of the data rate also indicates that the TPR is doubled. In the conventional mobile communication system, the data rate of a reverse traffic channel in relation to the TPR is preset in a table so that the BTS and the MS may recognize the relationship between the data rate of the reverse traffic channel and the TPR. Thus, a control for the data rate of the MS is substantially identical to a control for the TPR of the MS. In the following description, only a procedure of controlling the data rate of the MS by means of the BTS will be described for the purpose of convenience of explanation. However, as mentioned above, it is noted that a control scheme of the present invention is also applicable when the BTS controls the TPR instated of controlling the data rate of the MS.
A conventional MS used for autonomous transmission can transmit reverse data with various data rates within the TPR allocated with a maximum autonomous rate. The BTS transmits a grant message (hereinafter, referred to as “Grant” i.e., a control of reverse rate) to the MS in order to allow the MS to transmit the data with a predetermined data rate lower than the maximum autonomous rate, thereby limiting the maximum autonomous rate of the MS. According to the prior art, upon receiving the Grant from the BTS, the MS transmits data with the predetermined data rate at a point of data transmission time corresponding to the Grant, while transmitting next data using resources corresponding to the maximum autonomous rate. In addition, since the maximum autonomous rate can be changed only through a signaling message, a relatively long time corresponding to hundreds of microseconds is necessary in order to allocate a new maximum autonomous rate.
Under an actual reverse data transmission environment, the BTS may sufficiently allocate the maximum autonomous rate to a plurality of MSs. In this state, if service data having a higher priority are newly generated, the sum of the maximum autonomous rates is so large that resources to be allocated to the new service data may be insufficient. In this case, the maximum autonomous rate of each MS must be lowered in order to allocate the resources for the new service data. Current technologies require a long time for adjusting the maximum autonomous rate, so an additional transmission delay may occur when allocating the resources for the new service data, thereby degrading quality of the service. In addition, when limiting the maximum reverse autonomous rate by using the Grant, the Grant must be continuously transmitted to the MSs while the service having the higher priority is being transmitted. Thus, grant channels become complicated and the Grant cannot be transmitted to MSs receiving the same channels. In order to solve the above problem, a higher autonomous rate can be allocated for the service having the higher priority or users. In this case, however, the resources used for the reverse data transmission may be wasted during an interval in which the service data are not generated.
FIG. 1A is a diagram illustrating a conventional method of controlling a reverse rate, wherein
“A” is a forward control channel for transmitting the Grant from the BTS and “B” is a reverse packet data channel for transmitting data to the BTS. The Grant is information related to the data rate lower than the maximum reverse autonomous rate. When the BTS transmits the Grant to the MS at a point of T1 so as to limit the maximum autonomous rate, the MS determines the Grant as a temporal limitation signal for the data rate, so the MS allocates the data rate corresponding to the Grant only at a point of T2, which is a first transmission time for data. That is, the data rate lower than the maximum autonomous rate is allocated based on the Grant only at a first slot receiving the Grant. In this case, if the new service or the MS having a higher priority continuously requires the resources, the Grant must be continuously transmitted so as to limit the autonomous rate. That is, the BTS transmits the Grant to the MS in each point of transmission time (T1 to T4) in order to limit the autonomous rate. Accordingly, the grant channels become complicated, thereby causing a forward overhead. For this reason, it is difficult to easily control the reverse rate.